There are two concentric spheres of radii 3 units and 6 units. What is the volume, in cubic units, of the region within the larger sphere and not within the smaller sphere? Express your answer in terms of $\pi$.
Answer: The smaller one has volume $\frac43\cdot27\pi=36\pi$ cubic units and the larger one $\frac43\cdot216\pi=288\pi$ cubic units. The volume between them is the difference of their volumes, or $288\pi-36\pi=\boxed{252\pi}$ cubic units.